Bipolar transistors, BJTs are often used to provide simple and low cost active devices in RF mixers / multipliers..

Bipolar transistor mixers, BJT mixers tend to be found in low cost receivers, like portable radios that use discrete components.

Sometimes bipolar transistor mixers are found in radio receiver ICs where they provide the some conversion gain, although the linearity is poor.

As a result, single bipolar transistor RF mixers tend to be used in applications where performance requirements are not particularly high and where cost is more important.

Basic BJT transistor mixers

It is possible utilise a variety of different circuit configurations for a transistor mixer. Possibly the most obvious method is to apply both signals to the base of the transistor.

Basic bipolar transistor mixer circuit

In this circuit the filter on the output is required to remove any of the high frequency LO and RF signals. Typically this circuit will be used to convert signals from a high frequency down to a much lower frequency. If the filter consists of a capacitor this can be chosen to present a short circuit to the LO and RF inputs, whilst not affecting the IF signals.

Additionally the tuned circuits on the input prevent the LO and RF signals coupling in to the opposite sources.

A more common circuit for a transistor mixer applies the local oscillator to the base and the RF input to the emitter of the transistor.

Bipolar transistor mixer circuit with RF input applied to emitter

It is this transistor mixer circuit that forms the basis of many circuits within transistor radios using discrete transistors. Often a self oscillating mixer is used, where a single transistor circuit based around this configuration acts as an oscillator and mixer.

RF front end circuit from a transistor radio with self oscillating mixer

Other transistor mixer configurations

Although transistor mixers built around a single transistor would not be expected to provide the highest level of performance, however it is possible to considerably improve on the performance of a single bipolar transistor mixer by using more advanced circuit topologies.

One form of transistor mixer that works very well is known as the Gilbert cell and this is addresses on a separate page.

Nevertheless, the bipolar transistor frequency mixer has found a considerable amount of use and has performed well, offering acceptable performance at a very low cost. It was for this reason that it was widely used.